fluid control valves

7/30/2019 Fluid Control Valves

http://slidepdf.com/reader/full/fluid-control-valves 1/41

MH206- Fluid power Control

Valves



Hydraulic Valves

• Responsible for controlling and directingflow

– Directional flow control valves• Spool and internal passages dictate flow paths

– Proportional flow control valves

• Variable orifice

• Gate, globe, and needle valves

– Check valves (similar to a diode in an electricalcircuit)



1. Direction Controls

• Simple direction control

– On-off type direction control

• Multi-direction Control

– Circuits Switching-type direction control

• Proportional direction control

– Direction and flow integrated control



Check Valves



Directional Control Valves



Simple direction Control

2 1

Pp P

P2

P1

PT T P

T

Qk Qk

Pp P

P2 P

1

PT T P

T QP

Q1 Q2

QT

Pp P

2 1P2 P

1

PT T P

T QP

Q1 Q2

QT



• Above arecommon center-

spoolarrangements formatching neutral-position fluid

routes to theapplication



• This cutaway viewof a multiple-

spool stack valveshows maindirectional spools,internal flow

passages, andauxiliary valves



Solenoid actuated DCV



• Operators for directional-control valves areeither mechanical, pilot, electrical andelectronic, or a combination of these

• With a mechanical operator, a machineelement or person applies force on the valve'sflow-directing element to move or shift it toanother position. Manual operators includelevers, palm buttons, push buttons, andpedals. Purely mechanical operators includecams, rollers, levers, springs, stems, andscrews. Springs are used in most directional

valves to hold the flow-directing element in aneutral position



• Pilot-actuated valves are shifted bypressurized fluid (air or oil) that applies forceto a piston that shifts the valve's flow-

directing elements. An important advantage of pilot operation is that large shifting forces canbe developed without the impact and wearthat affects mechanically actuated valves.

• Electrical or electronic valve operation involvesenergizing a solenoid. The force generated atthe solenoid plunger then shifts the valve'sflow-directing element. Solenoid-actuated

valves are particularly popular for industrialmachines because of the ready availability of

electric power in industrial plants



Multi-direction Control

Case 1: Hydraulic Cylinder Controls

M

M

P

T

R

H

M

M

P

T

R

H

M

M

P

T

R

H

M

M

P

T

R

H

M

M

P

T

R

H



2. Pressure Controls

• System Pressure Control

– Use of a line relief valve to limit the maximum

allowable system pressure

• Other Pressure Control Functions

– Unloading

– Pressure reduction

– Load holding – Sequential control

– Others



Uses of pressure control valves

• Limit system pressure

• Reduce pressures

• Set pressure at which oil enters a circuit

• Unload a pump



Pressure-control valves• Pressure-control valves are found in virtually

every hydraulic system, and they assist in avariety of functions, from keeping systempressures safely below a desired upper limit tomaintaining a set pressure in part of a circuit.

• Types include relief, reducing, sequence,counterbalance, and unloading. All of theseare normally closed valves, except forreducing valves, which are normally open.



Line Relief Valve

Line relief valve is a pressure control valve.

A basic LR valve has a pilot pressure applied.

There are a few types of LR valves available.

Inlet

OutletPilot



Cracking pressure andpressure override

• Cracking pressure and pressure override - Thepressure at which a relief valve first opens to allow fluidto flow through is known as cracking pressure. Whenthe valve is bypassing its full rated flow, it is in a stateof full-flow pressure. The difference between full-flow

and cracking pressure is sometimes known as pressuredifferential , also known as pressure override.

• In some cases, this pressure override is notobjectionable. However, it can be a disadvantage if itwastes power (because of the fluid lost through the

valve before reaching the maximum setting). This canfurther permit maximum system pressure to exceed theratings of other components. (To minimize override, usea pilot-operated relief valve.)



Pressure Override

Characteristics

Flow

P r e s s u r e

QQmaxmax

PP

PPcrackingcracking

PPfullfull flowflow

Flow

P r e s s u r e

QQmaxmax

PP

PPcrackingcracking

PPfullfull flowflow



Improved Pressure Override

Characteristics

Flow

P r e s s u r e

QQmaxmax

PP

PPcrackingcracking

PPfullfull flowflow

Flow

P r e s s u r e

QQmaxmax

PP

PPcrackingcracking

PPfullfull flowflow

P P P P P P full full flow flow P P full full flow flow



Pressure Relief Valves• Found in all hydraulic circuits

• Normally closed

• Can be pilot operated (i.e. Spool is biased

by both spring and fluid pressure…less

override)

• Provides protection against exceeding

system pressure rating which can damagecomponents



Pressure Relief Valves



Pilot Operated Relief ValvesFunction: protect a hydraulic system with low override

characteristics.

Operation: The spring loaded puppet valve opens when thepressure reaches the cracking pressure, the flowcause the back pressure drop, which leads the openof the spool valve.



Compound pressure relief valve

• The pilot-operated relief valve operates in twostages.

• A pilot stage, which

consists of a small,spring-biased relief valve(generally built into themain relief valve), actsas a trigger to control themain relief valve.

• However, the pilot mayalso be located remotelyand connected to themain valve with pipe or

tubing.



Application of Relief Valves

M

M

P

T

R

H1,000 psi

800 psi

800 psi

It is recommended that there is always at least 200 psi differencebetween two line release (pressure relief) valves in a line.



Pressure Reducing valve



Pressure-reducingvalves

• The most practical components formaintaining secondary, lower pressure

in a hydraulic system are pressure-reducing valves. Pressure-reducingvalves are normally open, 2-way valvesthat close when subjected to sufficient

downstream pressure. There are twotypes: direct acting and pilot operated



Direct acting pressure reducing valve

• A pressure-reducing valve limits the maximumpressure available in the secondary circuitregardless of pressure changes in the main circuitand as long as the work load generates no backflow into the reducing valve port in which case thevalve will close

• The pressure-sensing signal comes from thedownstream side (secondary circuit). This valve, ineffect, operates in reverse fashion from a relief valve (which senses pressure from the inlet and isnormally closed).

• As pressure rises in the secondary circuit,

hydraulic force acts on area A of the valve, closingit partly. Spring force opposes the hydraulic force,so that only enough oil flows past the valve tosupply the secondary circuit at the desiredpressure. The spring setting is adjustable.



• When outlet pressure reaches that of the valve setting, the valve closesexcept for a small quantity of oil thatbleeds from the low-pressure side of the valve, usually through an orifice inthe spool, through the spring chamber,to reservoir.

• Should the valve close fully, leakagepast the spool could cause pressurebuild-up in the secondary circuit. Toavoid this, a bleed passage to reservoirkeeps it slightly open, preventing a risein downstream pressure above thevalve setting. The drain passage returnsleakage flow to reservoir.



Sequence valves

• Sequence valve isa 2-way valve

held closed by anadjustable springand opened bypressure at the

inlet port actingon the left of thespool



Sequence valves• Sequence valves are normally closed, 2-way

valves. They regulate the sequence in whichvarious functions in a circuit occur,

• They resemble direct-acting relief valves exceptthat their spring chambers are generally drainedexternally to reservoir, instead of internally to the

outlet port, as in a relief valve.• When the primary function is satisfied, pressure in

the primary circuit rises and is sensed in pressure-sensing passage A. This pressurizes the spool and

overcomes the force exerted by the spring. Thespring is compressed, the valve spool shifts, andoil flows to the secondary circuit.



Example Other Pressure Controls:

-- Sequence Valves



Counter balance Valve

• Counterbalance valve

stops flow from its inlet

port to its outlet port until

pressure at the inlet portovercomes adjusting

spring force. An integral

check valve permits free

flow through the valve inthe opposite direction.



• These normally-closed valves are primarily used tomaintain a set pressure in part of a circuit, usually to

counterbalance a weight or external force or counteract aweight such as a platen or a press and keep it from free-falling. The valve's primary port is connected to thecylinder's rod end, and the secondary port to the directionalcontrol valve. The pressure setting is slightly higher than

that required to keep the load from free-falling.

• When pressure fluid flows to the cylinder's cap end, thecylinder extends, increasing pressure in the rod end, andshifting the main spool in the counterbalance valve. Thiscreates a path which permits fluid to flow through thesecondary port to the directional control valve and toreservoir. As the load is raised, the integral check valveopens to allow the cylinder to retract freely.



Unloading Valve

• Unloading valve is spring-

loaded to the closed

position. When system

pressure, transmitted tothe valve though the pilot

port, is sufficient to

overcome force of the

adjustable spring, thevalve opens. Pump

delivery unloads to tank at

low pressure.



Unloading valves• These valves are normally used to unload pumps.

They direct pump output flow (often the output of oneof the pumps in a multi-pump system) directly toreservoir at low pressure, after system pressure has

been reached.

• The force exerted by the spring keeps the valve

closed, When an external pilot signal acting on theopposite end of the valve spool exerts a force largeenough to exceed that exerted by the spring, the valvespool shifts, diverting pump output to reservoir at low

pressure.

• High-low circuits which use two pumps for traverseand speed, or clamping, depend on unloading valvesto improve efficiency. Output from both pumps isneeded only for fast traverse. During feed or clamping, output from the large pump is unloaded to

reservoir at low pressure.



2. Flow Controls

• Uncompensated Flow Control

– Example – metering-in/metering-out using

needle valves • Compensated Flow Control

– Example -

• Proportional Flow Control

– Often use of proportional directional controlvalve (discuss in direction control)



Metering Controls of Flow

• Often us of a needle valve

• Simple, but energy inefficient• Commonly used in some light load, inexpensive systems

for load control.



Proportional Flow Control

Valves



Typical Metering Flow Control

Approaches



Flow Divider –Example of

Pressure-compensated Flow Control



Functionality and Operation

of a Flow Divider Valve• Directs flow to two or more branches

• Controls flow and also direction

(e.g., priority control)

• Common on off-road equipment to divide

flow between circuits

(e.g., power steering and front-end load or left and rightcrawler tracks)

fluid control valves

Documents